High quality lube base stock oils are conveniently prepared by refining the residuum prepared by the vacuum distillation of a suitable crude oil from which the lighter portion has been removed by distillation in an atmospheric tower.
The vacuum distillate fractions are upgraded by a sequence of unit operations, the first of which is solvent extraction with a solvent which is selective for aromatic hydrocarbons. This step serves to remove aromatic hydrocarbons of low viscosity index and provides a raffinate of improved viscosity index and quality. Various processes have been used in this extraction stage, and these employ solvents such as furfural, phenol, sulfur dioxide, and others. The vacuum tower residuum, because it contains most of the asphaltenes of the crude oil, is conventionally treated to remove these asphalt-like constituents prior to solvent extraction to increase the viscosity index.
The raffinate from the solvent extraction step contains paraffins which adversely affect the pour point. Thus, the waxy raffinate, regardless of whether prepared from a distillate fraction or from the vacuum tower residuum, must be dewaxed. Various dewaxing procedures have been used such as solvent dewaxing with the aid of a solvent such as MEK/toluene, propane dewaxing or catalytic dewaxing, as described in the Oil and Gas Fournal, Jan. 6, 1975, pages 69-73 and U.S. Pat. Nos. 3,668,113, 3,894,938, 4,137,148 and RE 28,398.
Various other processes have also been used to improve the qualities of lube oil and other base stocks, for example, by removing sulfur, nitrogen and oxygen compounds and by saturating aromatics to produce hydrocarbons with more desirable properties. One of the processes which has achieved widespread use in the petroluem refining industry for this purpose is hydrotreating or, as it is often referred to, hydrorefining or hydrofinishing. In hydrotreating, the feedstock which is to be treated is passed over a bifunctional catalyst which has both acidic and hydrogenation activity in the presence of hydrogen at elevated temperatures and pressures. Heterocyclic compounds containing sulfur, nitrogen and oxygen are subjected to ring opening and the impurities are converted to inorganic hydrogenated compounds which are removed as gases. Hydrogenation of unsaturated hydrocarbons also takes place, together with a mild degree of cracking. Removal of metals may also be accomplished by this treatment. Hydrotreating processes of various kinds are described, for example, in U.S. Pat. Nos. 4,137,148, 3,562,145, 3,853,749, 3,904,513, 3,985,643, 3,979,279 and 4,181,598.
The catalysts which have conventionally been used for hydrotreating processes have been large pore materials such as alumina, as described in U.S. Pat. Nos. 3,904,513 and 4,181,598, with a hydrogenation component such as nickel, cobalt or molybdenum deposited on it.